Filter aids are widely used in industrial processes for the filtration of various types of liquid mixtures containing solids in order to remove the solids therefrom. A filter aid typically exists in a solid dry powdered form, with the material and the particle size of the powder lending particular filtration characteristics to the filter aid. In many applications, particular filter aid materials are selected because they provide certain desired filtration characteristics.
The filtration characteristics of a filter aid are usually measured by a number of filtration parameters, including the permeability coefficient of the filter aid (i.e., the degree of porosity of a cake of the filter aid) expressed in "darcies", the wet bulk density expressed in grams per cubic centimeter, and the volumetric flow rate of water through the filter aid expressed in milliliters per second. Producers of filter aids frequently measure the filtration parameters of samples of the filter aids as part of a quality control procedure, so that the end user is assured of obtaining a filter aid having known filtration parameters. In addition, end users of filter aids often measure the filtration parameters of their filter aids as part of their own quality control processes.
Commonly, manual laboratory procedures are used for the determination of filtration parameters of filter aids. Such manual procedures require a laboratory technician to use a significant number of pieces of equipment and to accurately follow a predetermined series of steps and accurately measure and record data manually. For instance, the permeability of a filter cake typically is determined with the aid of a permeameter, which is a vertical cylindrical tube having a septum consisting of a stainless steel screen on which is placed a piece of filter paper to prevent a filter aid material from passing through so that a cake of the filter aid forms on the septum when a slurry of the filter aid is passed through the permeameter. The permeameter has a top cap for sealing the upper inlet port of the tube. The top cap has a pressure valve which is coupled to a pressure-regulated air supply. A bottom cap seals the lower end of the tube below the septum, and has a filtrate valve which can be opened to allow filtrate to exit the permeameter. The permeameter includes a differential pressure gauge which measures the pressure difference across the septum.
A typical manual procedure for determining the permeability of a filter aid with a permeameter is as follows: The technician weighs a sample of the filter aid and records the weight, and then transfers the sample to an Ehrlenmeyer flask and adds water to the flask to form a slurry. The technician then opens the inlet port of the permeameter and pours the sample slurry into the inlet port, and rinses the flask with additional water and adds the additional water to the inlet port to be sure all of the sample is transferred to the permeameter. The inlet port is then tightly closed and the pressure valve is opened to pressurize the permeameter. The air supply is adjusted to a predetermined pressure. The technician then must quickly open the filtrate valve and allow the water to drain to a level just above the filter cake which has formed on the septum. The filtrate is collected in the flask. The filtrate valve and pressure valves are then closed, and the inlet port is opened. The technician adds some additional water to the filtrate in the flask, and uses a funnel with a dispersion tube to transfer the water in the flask into the inlet port, being careful not to disturb the surface of the cake. Next, the inlet port is closed and the pressure valve is opened to pressurize the permeameter. The filtrate valve is opened, and after a steady flowrate is established, the technician begins collecting the filtrate in a graduated cylinder and begins timing with a stopwatch at the same moment. At that same moment, the technician must note the gauge pressure on the differential pressure gauge. The technician collects a certain amount of filtrate, and then removes the graduated cylinder and stops timing at the same moment. The technician must then note the gauge pressure again. A thermometer is used to measure the temperature of the filtrate. The permeameter is then completely drained of water, and the technician measures the thickness of the cake. Permeability of the filter aid is then calculated by the equation: ##EQU1## where V=volume of filtrate collected (ml)
n=viscosity (centipoise), calculated from temperature PA1 H=cake thickness (cm) PA1 t=time required to collect filtrate (sec) PA1 A=pressure chamber cross-sectional area (cm.sup.2) PA1 P=pressure differential (psi)
This manual procedure is time-consuming, and is inconvenient to perform in certain environments because of the substantial amount of equipment required. Furthermore, the procedure is subject to inaccuracy and inconsistent results because of the dependence on the technician manually running all phases of the experiment and reading and recording data.